1. Field of the Invention
Apparatuses and methods consistent with the present invention relate to correcting the power factor of input power, and more particularly, to correcting the power factor of input power in a switching mode power supply (SMPS).
2. Description of the Related Art
Related art power supply units are not suitable for household and industrial apparatuses having a small size and weight and a large capacity because the Related art power supply units have a limited capacity and a considerably large size and weight. Accordingly, an SMPS has been proposed.
The SMPS is widely used for an AC/DC converter, a DC/DC converter, a power supply unit for notebook computers and a power supply unit for mobile phones. The SMPS employs a power factor improvement circuit in order to increase the power efficiency. The power factor improvement circuit removes harmonics of an input current and makes the phases of the current and voltage of an input power correspond to each other to improve the power efficiency when an AC power is applied as the input power.
The power factor improvement circuit uses a storage element such as a capacitor or an inductor, which has a rated capacity with respect to an allowable current and voltage. When a current or a voltage that exceeds the rated capacity is applied to the capacitor or the inductor, the capacitor or the inductor loses magnetism or a dielectric thereof is damaged, and thus the capacitor or the inductor loses its operational function.
FIG. 1 is a block diagram of a related art power factor improving apparatus. Referring to FIG. 1, the related art power factor improving apparatus includes an input unit 110, a rectifier 120, a power supply unit 130, a power factor correction unit 140, and a converter 150.
The input unit 110 receives an AC power. The rectifier 120 full-wave-rectifies the AC power input to the input unit 110. The rectifier 120 is configured in the form of a bridge circuit using four diodes.
The power supply unit 130 receives the AC power and outputs a DC voltage for operating the power factor correction unit 140. The voltage output from the power supply unit 130 can be a standby voltage.
The power factor correction unit 140 improves the power factor of the AC power input to the input unit 110 and includes a signal generator 142, a switch 144, an inductor 146, a diode 147 and a capacitor 148.
A DC output voltage 141 of the power supply unit 130 is applied to the signal generator 142 of the power factor correction unit 140 to operate the signal generator 142. When a DC output voltage 141 higher than a predetermined threshold is applied to the signal generator 142, the signal generator 142 outputs a control signal 143 for improving the power factor of the AC power input to the input unit 110. The control signal 143 is a pulse signal and the pulse width of the control signal 143 is adjusted to control the operation of the switch 144. The pulse width of the control signal 143 is controlled such that the capacitor 148 stably maintains a target DC voltage with reference to various signals including the input power and a feedback signal 149 from the voltage of the capacitor 148.
The control signal 143 output from the signal generator 142 operates the switch 144. The switch 144 can be configured in the form of a field effect transistor (FET). The control signal 143 is applied to the gate of the FET. When the control signal 143 is in an ON state, the FET operates and thus the switch 144 is closed. Accordingly, a current path is formed through the input unit 110, the inductor 146 and the FET 144. When the control signal 143 is in an OFF state, the FET does not operate and thus the switch 144 is opened. Accordingly, a current path is formed through the input unit 110, the inductor 146, the diode 147 and the capacitor 148.
When the control signal 143 is in an ON state and thus the switch 144 is closed, energy from the input AC power is charged in the inductor 146. When the control signal 143 is in an OFF state and thus the switch 144 is opened, the capacitor 148 is charged with a voltage using the energy charged in the inductor 146. The converter 150 converts the voltage charged in the capacitor 148 into a desired target voltage and transfers the target voltage to a load.
FIG. 2 illustrates the operation of the related art power factor improving apparatus. (a) of FIG. 2 is a waveform diagram of an input AC voltage. Referring to (a) of FIG. 2, the input AC voltage is applied to the power factor improving apparatus for a time T0 through T1 and then the supply of the input AC voltage is cut for a time T2. Subsequently, the input AC voltage is applied again for a time T3 through T4. This frequent ON/OFF conversion of the input AC voltage can be caused by the power supply or a forced operation of a user.
(b) of FIG. 2 illustrates the output of the power supply unit 130. Referring to (b) of FIG. 2, when the input AC voltage is applied to the power supply unit 130, the power supply unit 130 converts the input AC voltage into a DC voltage and outputs a voltage Vcc 141 in order to operate the power factor correction unit 140.
When the input AC voltage is applied, the voltage Vcc is gradually increased and then maintained. The voltage Vcc 141 gradually varies because of a storage element included in the power supply unit 130, such as a capacitor or an inductor. When the supply of the input AC voltage is cut, the voltage Vcc 141 is gradually reduced to 0V. When the voltage Vcc 141 becomes lower than a predetermined threshold value, the operation of the power factor correction 140 is stopped.
During the period T0 through T1, the input AC voltage is applied to the power supply unit 130, and thus the power supply 130 outputs the DC voltage Vcc. The DC voltage Vcc is input to the power factor correction unit 140 to operate the power factor correction unit 140.
During the period T2, the supply of the input AC voltage is cut. However, the DC voltage Vcc output from the power supply unit 130 may be higher than a threshold voltage for a predetermined time even when the input AC voltage is not applied because energy stored in the storage element included in the power supply unit 130 gradually decreases with time. Accordingly, the power factor correction unit 140 continuously operates during the time when the DC voltage Vcc output from the power supply unit 130 is higher than the threshold voltage even when the AC input voltage is not applied for the period T2.
The DC voltage Vcc output from the power supply unit 130 gradually increases to a predetermined level for the period T3 through T4 during which the input AC voltage is applied again.
(d) of FIG. 2 illustrates the control signal 143 generated by the signal generator 142. The control signal 143 is a pulse signal that controls ON/OFF operations of the switch 144. The signal generator 142 receives the output voltage Vcc of the power supply unit 130, generates the control signal 143, and transfers the control signal 143 to the switch 144 only when the output voltage Vcc of the power supply unit 130 is higher than the predetermined threshold value. When the output voltage Vcc of the power supply unit 130 is lower than the predetermined threshold value, the signal generator 142 does not operate.
The signal generator 142 controls the pulse width of the control signal 143 on the basis of the input AC voltage and the output voltage 149 of the capacitor 148 such that the output voltage 149 of the capacitor 148 reaches a target voltage.
During the period T0, the pulse width of the control signal 143 generated by the signal generator 142 is gradually increased. The switch 144 is closed and thus the inductor 146 is charged when the control signal 143 is in an ON state while the switch 144 is opened and thus the energy charged in the inductor 146 is transferred to the capacitor 148 when the control signal 143 is in an OFF state.
If the control signal 143 of the signal generator 142 maintains the ON state for a long time, the switch 144 is closed for a long time and thus a large current flows to the inductor 146. The control signal 143 controls the output voltage 149 of the capacitor 148 to be maintained at a specific level based on the output voltage 149 of the capacitor 148 and the AC input voltage.
Since there is a large difference between the output voltage 149 of the capacitor 148 and the target voltage when the power factor correction unit 140 initially operates, the signal generator 142 generates the control signal 143 having a large pulse width such that the control signal 143 maintains the ON state for a long time. Accordingly, a large current flows to the inductor 146, and thus the inductor 146 is saturated. Consequently, the switch 144 is damaged and the power factor correction unit 140 cannot normally operate.
To solve this problem, the signal generator 142 can perform a soft-start operation that gradually increases a time for which the switch 144 is closed or can perform a start-up operation that starts the switch 144 after the output voltage 149 of the capacitor 148 has increased to a specific level when the power factor correction unit 140 initially operates. In the case of the power factor improving apparatus illustrated in FIG. 1, the signal generator 142 selects the soft-start operation to generate the control signal 143 having a pulse width that gradually increases for the period T0 when the power factor correction unit 140 initially operates.
During the period T1, the input AC voltage is applied and the power supply unit 130 converts the input AC voltage into the DC voltage Vcc and the transfers the DC voltage Vcc to the signal generator 142, and thus the power factor correction unit 140 operates. During the period T1, the output signal 149 of the capacitor 148 stably maintains a specific value.
During the period T2, the supply of the input AC voltage is cut, and thus the output DC voltage Vcc of the power supply unit 130 decreases. Here, the output DC voltage Vcc gradually decreases due to the storage element included in the power supply unit 130, such as a capacitor or an inductor, as described above. The output DC voltage Vcc of the power supply unit 130 is still higher than the predetermined threshold value, and thus signal generator 142 continuously operates. However, the output voltage 149 of the capacitor 148 continuously decreases even when the control signal 143 of the signal generator 142 is in an ON state because the input AC voltage is not applied. The signal generator 142 receives the output voltage 149 of the capacitor 148 and determines that there is a large difference between the output voltage 149 of the capacitor 148 and the target voltage. Accordingly, the signal generator 142 generates the control signal 142 to have a large pulse width in order to charge the capacitor 148 with the target voltage.
At the period T3, the input AC voltage is applied again. When the time during which the input AC voltage is cut and then applied again is short, the output DC voltage Vcc of the power supply unit 130 does not become lower than the predetermined threshold value and gradually increases again.
The signal generator 142 receives the output voltage 149 of the capacitor 148 through feedback and determines that there is a large difference between the output voltage 149 of the capacitor 148 and the target voltage, and thus the signal generator 142 generates the control signal 143 having a large pulse width in order to charge the capacitor 148 with the target voltage. When the signal generator 142 generates the control signal 143 having a large pulse width, the current flowing through the inductor 146 increases. When a current that exceeds the rated capacity of the inductor flows through the inductor 146, the inductor 146 loses magnetism and is saturated. At the same time, a large current flows to the switch 144 and damages the switch 144.
After the period T4, the switch 144 is damaged, and thus the power factor correction unit 140 loses the function thereof and the output voltage 149 of the capacitor 148 decreases.
(d) of FIG. 2 illustrates the output voltage 149 of the capacitor 148. During the period T0, the output voltage 149 of the capacitor 148 gradually increases. As described above with reference to FIG. 1, the capacitor 148 is charged using the energy charged in the inductor 146 when the control signal 143 generated by the signal generator 142 is in an OFF state and the switch 144 is opened.
During the period T1, the input AC voltage is applied and the output voltage 149 of the capacitor 148 is stably maintained.
During the period T2 when the supply of the input AC voltage is cut, the output voltage 149 of the capacitor 148 gradually decreases because the input AC voltage is not applied even when the signal generator 142 operates.
At the period T3, the input AC voltage is applied again and a large current flows to the capacitor 148 because the signal generator 142 generates the control signal 143 that maintains an ON state for a long time. Accordingly, the output voltage 149 of the capacitor 148 rapidly increases. However, when the quantity of current flowing through the inductor 146 exceeds the rated capacity of the inductor 146, the inductor 146 loses magnetism and a large current flows to the switch 144 and damages the switch 144.
After the period T4, the switch 144 does not operate, and thus the power factor correction unit 140 does not operate and the output voltage 149 of the capacitor 148 gradually decreases.
To solve this problem, the related art power factor improving circuit uses an inductor having a high capacity or an inductor made of an expensive material such as a powder type magnetic substance that is difficult to saturate. This increases the size and cost of a power supply unit employing the power factor improving circuit.